1. Field of Use
This invention relates to data processing systems and more particularly to methods and apparatus for transferring different types of digital information between two devices for processing by them and for controlling their operation.
2. Prior Art
In systems in which a control device is located remotely from the device or devices it communicates with, a transmission network normally serves to interconnect these devices for transmitting and receiving information. The transmission network is usually by far the most expensive element in the system. This is particularly true when the network is required to accommodate the devices of a system which exchange different types of digital information for status and controlling purposes. An example of such a system is a cathode ray tube (CRT) display system in which a control device is required to service a plurality of display units. Normally, the device or "cluster" control unit is arranged to transmit and receive digital information to/from a data processing unit, as for example, a computer. The cluster control unit then processes the information, stores it, and thereafter forwards the information to the individual display units.
The "cluster" unit includes logic for editing the information it receives from each of the display units, storage for each of the display units, timing and character generation logic circuits for these units. In this arrangement, the transmission network is required to transmit control and video information from the "cluster" unit to display unit. And, where the display unit includes a keyboard for data entry, the transmission network is required to also transmit keyboard generated data and control signals from the display unit to the cluster unit.
Some prior art system have provided individual transmission networks for transmitting each of the different types of information. These systems have proved extremely expensive and normally require considerable space for housing the many multiconductor cables and lines.
Other prior art systems have employed a number of different DC voltage levels for each of the different types of information. These systems have found to be unsatisfactory in that they require threshold devices which are able to detect a number of different -levels. Also, these devices have been found to be less reliable where the device is required to recognize more than two different voltage levels. Specifically, when the circuits are required to recognize several different voltage levels, they become more susceptible to noise and other disturbances. This in turn renders the system more susceptible to errors and less reliable. Further, these systems have not been found to be readily adaptable for bidirectional transmission.
Accordingly, it is an object of the present invention to provide a technique for communicating different types of digital information.
It is therefore another object of the present invention to provide a transmission system of approved quality.
It is still another object of the present invention to provide a communication system which reduces the number of conductors required for transmitting different types of digital information signals and is highly reliable.
It is a more specific object of the present invention to provide a low cost system for interconnecting a pair of devices for bidirectional transfer of different types of digital information.
It is still a more specific object of the present invention to provide as arrangement for interconnecting a CRT control unit to a CRT display unit for bidirectional transmission of pulse coded video information, and control signals therebetween.